The precise positioning of organ progenitor cells constitutes an essential, yet poorly understood step during organogenesis. Using primordial germ cells that participate in gonad formation, we present the developmental mechanisms maintaining a motile progenitor cell population at the site where the organ develops. Employing high-resolution live-cell microscopy, we find that repulsive cues coupled with physical barriers confine the cells to the correct bilateral positions. This analysis revealed that cell polarity changes on interaction with the physical barrier and that the establishment of compact clusters involves increased cell-cell interaction time. Using particle-based simulations, we demonstrate the role of reflecting barriers, from which cells turn away on contact, and the importance of proper cell-cell adhesion level for maintaining the tight cell clusters and their correct positioning at the target region. The combination of these developmental and cellular mechanisms prevents organ fusion, controls organ positioning and is thus critical for its proper function.

Pitt-Hopkins syndrome (PTHS) is a neurodevelopmental disorder caused by monoallelic mutation or deletion in the () gene. Individuals with PTHS typically present in the first year of life with developmental delay and exhibit intellectual disability, lack of speech, and motor incoordination. There are no effective treatments available for PTHS, but the root cause of the disorder, haploinsufficiency, suggests that it could be treated by normalizing gene expression. Here, we performed proof-of-concept viral gene therapy experiments using a conditional mouse model of PTHS and found that postnatally reinstating expression in neurons improved anxiety-like behavior, activity levels, innate behaviors, and memory. Postnatal reinstatement also partially corrected EEG abnormalities, which we characterized here for the first time, and the expression of key TCF4-regulated genes. Our results support a genetic normalization approach as a treatment strategy for PTHS, and possibly other TCF4-linked disorders.

Super-resolution microscopy (SRM) is gaining popularity in biosciences; however, claims about optical resolution are contested and often misleading. In this Viewpoint, experts share their views on resolution and common trade-offs, such as labelling and post-processing, aiming to clarify them for biologists and facilitate deeper understanding and best use of SRM.

Glomeruli in the vertebrate olfactory bulb (OB) appear as anatomically discrete modules receiving direct input from the olfactory epithelium (OE) via axons of olfactory receptor neurons (ORNs). The response profiles with respect to amino acids (AAs) of a large number of ORNs in larval Xenopus laevis have been recently determined and analysed. Here we report on Ca(2+) imaging experiments in a nose-brain preparation of the same species at the same developmental stages. We recorded responses to AAs of glomeruli in the OB and determined the response profiles to AAs of individual glomeruli. We describe the general features of AA-responsive glomeruli and compare their response profiles to AAs with those of ORNs obtained in our previous study. A large number of past studies have focused either on odorant responses in the OE or on odorant-induced responses in the OB. However, a thorough comparison of odorant-induced responses of both stages, ORNs and glomeruli of the same species is as yet lacking. The glomerular response profiles reported herein markedly differ from the previously obtained response profiles of ORNs in that glomeruli clearly have narrower selectivity profiles than ORNs. We discuss possible explanations for the different selectivity profiles of glomeruli and ORNs in the context of the development of the olfactory map.

In a recent Editorial, De Schutter commented on our recent study on the roles of a cortico-cerebellar loop in motor planning in mice (De Schutter 2019, Neuroinformatics, 17, 181-183, Gao et al. 2018, Nature, 563, 113-116). Two issues were raised. First, De Schutter questions the involvement of the fastigial nucleus in motor planning, rather than the dentate nucleus, given previous anatomical studies in non-human primates. Second, De Schutter suggests that our study design did not delineate different components of the behavior and the fastigial nucleus might play roles in sensory discrimination rather than motor planning. These comments are based on anatomical studies in other species and homology-based arguments and ignore key anatomical data and neurophysiological experiments from our study. Here we outline our interpretation of existing data and point out gaps in knowledge where future studies are needed.

Action potentials are the end product of synaptic integration, a process influenced by resting and active neuronal membrane properties. Diversity in these properties contributes to specialized mechanisms of synaptic integration and action potential firing, which are likely to be of functional significance within neural circuits. In the hippocampus, the majority of subicular pyramidal neurons fire high-frequency bursts of action potentials, whereas CA1 pyramidal neurons exhibit regular spiking behavior when subjected to direct somatic current injection. Using patch-clamp recordings from morphologically identified neurons in hippocampal slices, we analyzed and compared the resting and active membrane properties of pyramidal neurons in the subiculum and CA1 regions of the hippocampus. In response to direct somatic current injection, three subicular firing types were identified (regular spiking, weak bursting, and strong bursting), while all CA1 neurons were regular spiking. Within subiculum strong bursting neurons were found preferentially further away from the CA1 subregion. Input resistance (R(N)), membrane time constant (tau(m)), and depolarizing "sag" in response to hyperpolarizing current pulses were similar in all subicular neurons, while R(N) and tau(m) were significantly larger in CA1 neurons. The first spike of all subicular neurons exhibited similar action potential properties; CA1 action potentials exhibited faster rising rates, greater amplitudes, and wider half-widths than subicular action potentials. Therefore both the resting and active properties of CA1 pyramidal neurons are distinct from those of subicular neurons, which form a related class of neurons, differing in their propensity to burst. We also found that both regular spiking subicular and CA1 neurons could be transformed into a burst firing mode by application of a low concentration of 4-aminopyridine, suggesting that in both hippocampal subfields, firing properties are regulated by a slowly inactivating, D-type potassium current. The ability of all subicular pyramidal neurons to burst strengthens the notion that they form a single neuronal class, sharing a burst generating mechanism that is stronger in some cells than others.

Decapentaplegic (dpp) regulates many aspects of imaginal disc growth and patterning in Drosophila. We have analyzed the phenotype of an eye-specific dpp allele, dppblk, which causes a reduction in the size of the retina due to a loss of ventral ommatidia. Prior to the onset of differentiation, dppblk eye discs are normal regarding size, shape, and ability to express dorsal and ventral markers. However, expression of a dpp-lacZ reporter is reduced at the ventral margin. Additional dorsoventral asymmetry appears during retinal differentiation: the morphogenetic furrow (MF) initiates normally at the posterior tip of the disc, but fails to propagate into the ventral epithelium. This defect can be rescued by increasing dpp expression along the ventral margin by local removal of patched function. We propose that the primary defect in dppblk is an inability to activate dpp expression properly at the ventral margin. This has two consequences: it prevents initiation from the ventral margin, and it renders the ventral epithelium unresponsive to differentiation signals emanating from the MF.

Most neurons function in the context of pathways that process and propagate information through a series of stages, e.g., from the sensory periphery to cerebral cortex. Because activity at each stage of a neural pathway depends on connectivity at the preceding one, we hypothesized that during development, axonal output of a neuron may regulate synaptic development of its dendrites (i.e., retrograde plasticity). Within pathways, neurons often receive input from multiple partners and provide output to targets shared with other neurons (i.e., convergence). Converging axons can intermingle or occupy separate territories on target dendrites. Activity-dependent competition has been shown to bias target innervation by overlapping axons in several systems. By contrast, whether territorial axons or dendrites compete for targets and inputs, respectively, has not been tested. Here, we generate transgenic mice in which glutamate release from specific sets of retinal bipolar cells (BCs) is suppressed. We find that dendrites of silenced BCs recruit fewer inputs when their neighbors are active and that dendrites of active BCs recruit more inputs when their neighbors are silenced than either active or silenced BCs with equal neighbors. By contrast, axons of silenced BCs form fewer synapses with their targets, irrespective of the activity of their neighbors. These findings reveal that retrograde plasticity guides BC dendritic development in vivo and demonstrate that dendrites, but not territorial axons, in a convergent neural pathway engage in activity-dependent competition. We propose that at a population level, retrograde plasticity serves to maximize functional representation of inputs.

The functional features of neural circuits are determined by a combination of properties that range in scale from projections systems across the whole brain to molecular interactions at the synapse. The burgeoning field of neurocartography seeks to map these relevant features of brain structure—spanning a volume ∼20 orders of magnitude—to determine how neural circuits perform computations supporting cognitive function and complex behavior. Recent technological breakthroughs in tissue sample preparation, high-throughput electron microscopy imaging, and automated image analyses have produced the first visualizations of all synaptic connections between neurons of invertebrate model systems. However, the sheer size of the central nervous system in mammals implies that reconstruction of the first full brain maps at synaptic scale may not be feasible for decades. In this review, we outline existing and emerging technologies for neurocartography that complement electron microscopy-based strategies and are beginning to derive some basic organizing principles of circuit hodology at the mesoscale, microscale, and nanoscale. Specifically, we discuss how a host of light microscopy techniques including array tomography have been utilized to determine both long-range and subcellular organizing principles of synaptic connectivity. In addition, we discuss how new techniques, such as two-photon serial tomography of the entire mouse brain, have become attractive approaches to dissect the potential connectivity of defined cell types. Ultimately, principles derived from these techniques promise to facilitate a conceptual understanding of how connectomes, and neurocartography in general, can be effectively utilized toward reaching a mechanistic understanding of circuit function.

Behaviour is governed by activity in highly structured neural circuits. Genetically targeted sensors and switches facilitate measurement and manipulation of activity in vivo, linking activity in defined nodes of neural circuits to behaviour. Because of access to specific cell types, these molecular tools will have the largest impact in genetic model systems such as the mouse. Emerging assays of mouse behaviour are beginning to rival those of behaving monkeys in terms of stimulus and behavioural control. We predict that the confluence of new behavioural and molecular tools in the mouse will reveal the logic of complex mammalian circuits.